The present invention relates to an alumina sol, a process for producing the same, a process for producing an alumina forming product using the same, and an alumina supported catalyst obtained by the use of the alumina forming product. More particularly, the invention relates to an alumina sol comprising fibrous boehmite of high monodispersibility which contains a small amount of water of crystallization and is composed of thin and long boehmite fibers. The invention also relates to a process capable of producing the alumina sol in a high concentration. The invention further relates to a process for producing an alumina forming product, wherein the above alumina sol is used without subjecting it to a special concentration operation to produce an alumina forming product that is optimum for alumina adsorbent or alumina carrier which has excellent mechanical strength and sharp pore distribution and is useful in fields of catalyst industry, exhaust gas purification and petroleum chemistry. The invention furthermore relates to an alumina supported catalyst which is obtained by supporting an active metallic component on the alumina forming product and thereby shows desired properties, which has excellent mechanical strength and sharp pore distribution, and which is favorable for heavy oil treatment in the petroleum refining.
Processes for producing alumina sols, alumina methods having .rho.- and .chi.-crystal structures, hydrolyses of aluminum salts, metal supporting methods, forming methods and hydrogenation catalysts, etc. of the prior art, all relating to the present invention, are described below.
With respect to an aqueous alumina sol obtained by hydrolysis of aluminum alkoxide, Japanese Patent Laid-Open Publication No. 10535/1995 (referred to as "Publication No. 1" hereinafter) discloses a process wherein hydrated alumina is deflocculated under heating in the presence of an acid to produce a transparent low-viscosity alumina sol. With respect to needle-like colloidal boehmite, a process wherein aluminum alkoxide is hydrolyzed to produce boehmite having a length of 100 to 500 nm is disclosed in J. Am. Cera. Soc., 74(6) 1,303-1,307 (1991) (referred to as "Publication No. 2" hereinafter).
With respect to a process for producing an aqueous alumina sol using metallic aluminum, Japanese Patent Laid-Open Publication No. 166220/1985 (referred to as "Publication No. 3" hereinafter) discloses a process wherein an amorphous fibrous alumina sol is produced from metallic aluminum and an organic acid; and Japanese Patent Laid-Open Publication No. 24824/1993 (referred to as "Publication No. 4" hereinafter) discloses a process wherein hydrochloric acid is added in the presence of silicic acid ion and sulfuric acid radical to produce an amorphous alumina sol of bunchy fibers having a diameter of 20 to 100 .mu.m and a length of 200 to 500 .mu.m.
As examples of the processes using a starting material analogous to that of the present invention, that is, the processes for producing alumina forming products wherein ultrafine boehmite or bayerite is synthesized from alumina having .rho.- and .chi.-crystal structures obtained by the contact of aluminum hydroxide with hot gas and the boehmite or bayerite is used to produce alumina forming product, the following ones can be mentioned. Japanese Patent Publication No. 21319/1975 (referred to as "Publication No. 5" hereinafter) discloses a process for producing an active alumina forming product. Japanese Patent Laid-Open Publication No. 74994/1976 (referred to as "Publication No. 6" hereinafter) discloses a process for producing a forming product carrier for a high-purity, thermally stable, active alumina supported catalyst. Japanese Patent Publication No. 13446/1984 (referred to as "Publication No. 7" hereinafter) discloses a process for producing alumina in the form of flaky or bunchy ultrafine boehmite. Japanese Patent Laid-Open Publication No. 72624/1986 (referred to as "Publication No. 8" hereinafter) discloses a process for producing dispersible hydrated aluminum oxide having a low bulk density.
In the general process for producing an alumina forming product, aluminum salt, aluminate or the like is hydrolyzed to produce an alumina hydrogel in the presence of a salt as a by-product, and the hydrogel is subjected to washing of the salt and concentration to obtain an alumina forming product. In the forming product obtained by the process, the surface area and the pore volume can be made large, but the pore structure and the strength are insufficient. In such circumstances, Japanese Patent Laid-Open Publication No. 26512/1986 (referred to as "Publication No. 9" hereinafter) describes improvement of the pore structure. With respect to industrial forming, Japanese Patent Laid-Open Publications No. 7164/1971 and No. 16395/1972 (referred to as "Publication No. 10" and "Publication No. 11", respectively, hereinafter) disclose a process comprising washing an alumina hydrogel obtained by hydrolysis over a filter to remove a salt as a by-product, subjecting the hydrogel filter cake containing a large amount of water to spray drying in a hot gas to obtain a dry product, pulverizing the product, adjusting the water content and forming. Japanese Patent Laid-Open Publication No. 104568/1994 (referred to as "Publication No. 12" hereinafter) discloses a process comprising washing a hydrogel obtained by hydrolysis, drying the hydrogel, mixing the dry gel in a mixer in the presence of a deflocculating agent, and extruding the mixture.
The hydrogenation catalyst carrier for use in the petroleum refining industry is produced by the hydrolysis method capable of making the pore volume large, but the strength of the resulting catalyst is insufficient. In the petroleum refining industry, the insufficient strength of the catalyst may cause such a serious problem that powdering of the catalyst takes place in the reactor to mainly cause a biased stream, whereby a local abnormal high temperature is brought about.
In the three way catalysts for the automobile exhaust gas purification, catalyst carriers wherein materials mainly made of alumina are supported on honeycombs made of cordierite or stainless steel are employed. The corrugated catalyst carriers having a laminated structure are ideal in other catalytic reactions, but they have not been commercialized yet.
In the process for producing an alumina sol using aluminum alkoxide, that is described in Publication No. 1, an alumina sol is synthesized in an alumina concentration of 5 to 10%, and the alumina sol is concentrated by heating to have a concentration of up to 20%. The fiber form is not described, but from the viscosity formula of the colloidal solution, it is easily presumed that the resulting boehmite particle in the low-viscosity alumina sol is extremely short. The alumina forming product obtained from the particles has a three-dimensional network with narrow voids even when the alumina sol is converted to a hydrogel, and therefore it becomes difficult to ensure a sufficient pore volume. In the process of Publication No. 2, colloidal boehmite in the form of needles of 200 to 500 nm and having a low content of water of crystallization is obtained, but there remain problems such as that plural kinds of aluminum alkoxides are used, the synthesized alumina sol has only a low alumina concentration of not more than 1%, and because of the too low alumina concentration, extensive equipment and a large number of steps are necessary for conducting concentration. Moreover, there is a commercial problem such that the use of the special aluminum alkoxide and the synthesis conditions within the low-concentration region make it impossible to mass produce alumina sols at low costs.
In each of the processes of Publications No. 3 and No. 4 to produce a fibrous alumina sol from metallic aluminum, an alumina sol of amorphous fibrous particles is obtained. In the fibrous alumina sol obtained by the process described in Publication No. 4, the fibers are in the form of bunches. In such alumina sol of bunchy fibers, however, the pore diameter formed in the secondary particles (agglomerates) is small, and macropores are produced as voids among the secondary particles (agglomerates). Thus, the whole pore distribution becomes such a broad pore distribution that various pores from micropores to macropores are present, so that any favorable pore structure cannot be expected. The alumina concentrations in the examples are 10 to 11%, and therefore if a high concentration is desired, great concentration equipment is necessary.
The process for producing an alumina forming product, wherein starting alumina having .rho.- and .chi.-crystal structures is used and the crystal form is transformed to a desired crystal form, has more simple steps as compared with the hydrolysis method. However, the alumina forming products obtained in Publications No. 5 and No. 6 have a small pore volume and are insufficient in physical properties required for various uses such as catalysts. Further, it is not described in the publications that an alumina forming product having a sharp pore distribution was produced.
Publication No. 7 relates to a process for producing an aqueous suspension of alumina in the form of ultrafine boehmite by treating alumina obtained by rapid heating dehydration of aluminum hydroxide with a monobasic acid and another chemical agent. In this process, synthesis is carried out with stirring under the conditions of an anionic ion/alumina molar ratio of not more than 6 and a temperature of 120 to 225.degree. C. in the presence of an acid and a salt each having a pH value of not more than 9. As can be seen from the description in the specification and FIGS. 2 to 5, an aqueous suspension of ultrafine boehmite in the form of flakes or bunches is obtained in this process. In spite of the sol state, the ultrafine boehmite in the form of flakes or bunches has a large particle diameter and is precipitable. Moreover, it has great resistance to filtration and a low light transmittance, and is opaque or cloudy. When an alumina forming product is produced from the flaky boehmite, the forming product has a defect of a small pore volume of pores having a preferred pore diameter. On the other hand, the bunchy boehmite produces a forming product having a broad pore distribution wherein various pores from micropores to macropores are present and showing low strength because of presence of the macropores, similarly to the bunchy alumina sol described in the aforesaid Publication No. 4. Thus, conditions for synthesizing boehmite suitable for alumina adsorbent, alumina carrier or hydrogenation catalyst are not fixed in Publication No. 7. Publication No. 8 has such a problem that starting alumina having a mean particle diameter of 0.4 to 0.6 .mu.m is necessary and adjustment of the starting material needs extensive equipment.
The alumina forming products obtained by hydrolysis of aluminum salts or aluminates are widely used, for example, as hydrogenation catalysts for petroleum refining, but they have the following qualitative and productive problems.
An important qualitative problem of the hydrogenation catalysts produced by hydrolysis of aluminum salts is low strength. In order to precipitate aluminum hydroxide by neutralization reaction of an acid and alkali, the reaction must be carried out at a higher rate than the rate at which the boehmite crystal lattice is arranged in order and which is inherent in boehmite. Under such conditions, however, obtainable are only (1) disordered crystal lattice containing a large amount of water of crystallization, (2) short fibers and (3) secondary particles (agglomerates) in which fibers of primary particles are agglomerated. These basic particles have low strength, and drying shrinkage of the forming product is large owing to the particle form of the boehmite. As a result, fine cracks are produced in the drying stage, whereby the strength is lowered. Basically, the conditions of pH value and the temperature in the neutralization reaction in the hydrolysis method are those for the precipitation of gibbsite or bayerite. Therefore, under the reaction conditions suitable for a slow growth rate that is necessary for arranging the boehmite crystal lattice in order, gibbsite or bayerite is produced. That is, satisfactory reaction time conditions matching with the crystal growth rate inherent in the boehmite crystal cannot be determined.
For the above reasons, the aluminum salt hydrolysis method has such a problem that monodispersible boehmite comprising long fibers and having orderly crystal lattice cannot be obtained.
The hydrolysis method has other problems about its process, for example, problems of high costs and environment accompanied by washing of a salt as a by-product and problems of oversized equipment and plural stages accompanied by synthesis of a dilute product. As described in Publications No. 10 and No. 11, the process to obtain a forming product of boehmite produced by hydrolysis of aluminum salt comprises a step of preparing an aluminum salt aqueous solution, a step of hydrolysis by neutralization and a step of removing a salt as a by-product by washing with ion-exchange water, and further comprises additional plural steps of spray drying, pulverization, sieving and water content adjustment, prior to the extruding step, because the washed filter cake has a low alumina concentration. Thus, the process is very complicated and lacks productivity.
To improve insufficient strength of the hydrogenation catalysts, a method of using an alumina sol obtained by the aforesaid aluminum alkoxide method or metallic aluminum method has been hitherto proposed as a substitution of the aluminum salt hydrolysis method. In the method, however, it is impossible to form a suitable pore structure, or if possible, there reside other problems of cost and quality. Also in the case of alumina obtained by the use of an alumina starting material having .rho.- and .chi.-crystal structures, that is described in the above publications, it is impossible to form a suitable pore structure, or if possible, there reside other qualitative problems.
It is necessary to obtain boehmite primary particles which are so designed that they are capable of providing alumina adsorbent, alumina carrier or hydrogenation catalysts showing high strength with keeping a preferred pore volume and a sharp pore distribution, and to establish a process for synthesizing them.
It is known that the pore distribution of the catalyst carrier has close relation to catalytic activity, selectivity and life. Particularly in case of a hydrogenation catalyst for heavy oil, blocking of pores caused by diffusion of asphaltene in the heavy oil into pores causes deactivation of the catalyst. Therefore, there is desired a catalyst having a macropore-free pore structure, a moderately large pore volume, a small number of micropores for the purpose of decreasing decomposition activity, and a moderate specific surface area for the purpose of maintaining high activity and high mechanical strength. This proposes a basic subject of alumina material for constituting alumina adsorbent, alumina carrier or hydrogenation catalyst.
In order to allow an alumina carrier to have a large specific surface area, the alumina carrier should be made from boehmite having a small particle diameter, because the specific surface area depends on the outside surface area of the basic particles. It is a premise of a large pore volume that the alumina carrier is composed of a fibrous (or needle-like) material having a three-dimensional network. In order to allow an alumina carrier to have a sharp pore distribution, it is essential that the alumina carrier is constituted of primary particles monodispersed. Secondary particles (agglomerates) are unfavorable, because micropores are produced in the agglomerates and macropores are produced between the agglomerates. For obtaining an alumina forming product having high mechanical strength, it is desired that the inside structure of the crystal lattice of the primary particles is in order. For preventing cracks, it is desired that fibers having moderate thickness and length are monodispersed. Further, it is desired that production of macropores between the secondary particles (agglomerates) is minimized, because the macropores exert great influences on the mechanical strength.